Prior U.S. Pat. No. 4,139,639 to Bahoshy et al, describes encapsulating aspartame with gum arabic or dextrin using either spray drying or coacervation. In either process, the encapsulating material and aspartame are first formed into a water-based emulsion. The disclosure of this patent is remote from the concepts of the present invention.
It is known to encapsulate aspartame using a fluid bed-type process, as described in prior U.S. Pat. No. 4,384,004 to Cea et al. More specifically, the process employed in this patent is referred to as the Wurster process, and it is characterized as one that operates in a "similar fashion" to a fluidized bed coating process. Pretreatment for the aspartame, for instance pretreatment by compaction, is not disclosed or suggested in the patent. The coating material is described as a water soluble solution, dispersion or emulsion of a material like gum arabic, dextrin, cellulose derivatives, waxes, gelatin, zein and the like. Application is by finely atomizing the coating onto the aspartame particles.
Several "Wurster process" patents are referred to in the Cea et al patent. All appear to be somewhat different, and the Cea et al patent is not specific on which process is used. Reference is made to an Example II process in the above mentioned U.S. Pat. No. 4,139,639, but there is no Example II in this patent.
A commercially available form of the Wurster process or apparatus is one similar to FIG. 7 of Wurster U.S. Pat. No. 3,241,520. In this patent, aspouting bed is established within a cylindrical partition, with recirculation downwardly and back via an annulus around the partition. In large commercial units, a plurality of these cylindrical partitions and accompanying spray nozzles are employed. A spray nozzle is centered and positioned beneath the partion. Whereas this process and apparatus may function well with application of a water-soluble dextrin or gum arabic, or other solvent coating, to the aspartame, it is more difficult, for a number of reasons, for encapsulation by a melted hydrocarbon or wax, such as a partially hydrogenated vegetable oil stearine.
For one, the use of multiple spray nozzles in the larger production units to spray apply a melted hydrocarbon or wax into a fluid bed is impractical. Should one of the nozzles plug, this could occur without the operator knowing it, resulting in uneven application of coating.
Powders, such as aspartame, possess a needle-like crystal structure which causes them to "snowball" when placed in a conventional fluid bed reactor. In this phenomenon, the crystals tend to intertwine or cling together as they fluidize, causing the formation of soft, fibrous, aggregate balls of aspartame.
Although the crystals can easily be broken up with minimal direct mechanical pressure, the relatively gentle motion of the fluid bed induces the aggregates to grow unchecked, resulting in disruption of necessary fluid bed movement.
Traditionally, a fluidized bed reactor can be used to dry, granulate, or encapsulate solid particulate material. The particulate material must be free flowing in the bed before it can be effectively processed. Powders that exhibit poor flow characteristics can often be rendered acceptable for fluid bed processing by the addition of free-flow agents such as silicon dioxide, tricalcium phosphate, magnesium oxide, or a number of other agents. Occasionally, the added free-flow agents do not significantly improve particle flow.
In the present instance, this was the case. Particle flow properties of aspartame in a fluidized bed were not improved by the addition of various amounts of either silicon dioxide or dicalcium phosphate, two common free-flow agents. In addition, low bulk density starch bulking agents such as maltodextrin were unsuccessfully employed to improve the flow properties of aspartame in a fluid bed.
There are other crystalline products that in a natural state have a needle-like structure and, thus, as with aspartame, are difficult to encapsulate. One example is niacinamide, a nutritional supplement, which has the same detrimental flow characteristics as aspartame. Another example is Ibuprofen, an anti-inflammatory drug (Merck Index, Tenth Edition, Monograph 4797).